The thermoforming process may be used to form plastic articles of all sizes, and is particularly effective when it is desired to form large plastic articles at low cost. Articles thermoformed from a single sheet of thermoplastic material are generally limited in wall thickness to the thickness of the sheet or less, throughout. By utilizing a twin-sheet thermoforming process articles of greater thickness and stiffness may be formed. The overall thickness of an article formed from twin-sheets may be varied by providing a separation between the thermoformed sheets. However, solid portions of the article are limited to the combined thicknesses of the two fused sheets from which the article is formed. Also, portions of the article which are formed more deeply in the mold will tend to be thinner than the full combined thicknesses of the sheets from which the article is formed, because the sheets must be drawn and stretched, with corresponding thinning, to form such portions.
Thus, although it is possible to form large and very stiff articles using the twin-sheet thermoforming process, the strength of the articles is limited to the strength obtained by fusing the two sheets in various configurations. Generally, if it is desired or necessary to increase the strength of a portion of a formed twin-sheet article, it is necessary to increase the thickness of one or both of the entire sheets, which results in higher overall cost of material. A need exists for a twin-sheet thermoformed article having reinforced portions which would stand up to greater wear or abrasion conditions, or facilitate efficient connection of such articles.
It has been common practice to reinforce plastic twin-sheet thermoformed articles with metal, wood, or other reinforcing material. These reinforcements may be costly and detract from the advantageous properties of an all-plastic article, such as low density, fire resistance, and weatherability. Also, the addition of non-plastic reinforcements makes eventual recycling of the article difficult as the non-plastic portions must be separated from the plastic before the plastic may be used again.
Methods for producing uniformly reinforced plastic sheet material wherein reinforcing strands or webs are positioned between bonded sheets are well known. A uniformly reinforced plastic sheet, however, results in an unnecessary increase in the overall weight and material usage of the finished article. A process has also been developed for selectively reinforcing portions of a single-sheet thermoformed article involving the application of powdered plastic to the sheet of thermoplastic material prior to drawing the sheet in the vacuum mold. This process requires complicated forms for retaining the reinforcing plastic on the plastic sheet during the heating step, and does not appear well adapted for twin-sheet thermoforming. Reinforcements employing plastic foams are also known, but articles formed of different types of plastic are also difficult to recycle.
An improved thermoformed article and thermoforming process are needed which retain advantages and efficiencies of twin-sheet thermoforming yet allow localized increases in thickness of the finished article.